Photopolymerization or "UV curing" offers a rapid, environmentally compatible and economically attractive method for preparing three dimensional polymer networks. Consequently, UV curing has been widely used for thin film applications such as coatings, inks, and adhesives. Due to the development of diaryliodonium and diarylsulfonium salts as two classes of practical cationic photoinitiators, this area of polymer photochemistry has enjoyed rapid development over the past two decades and has been applied to the polymerization of a wide range of monomer types.
One of the challenges in polymer chemistry is to develop such polymeric materials from inexpensive, environmentally compatible and renewable sources of starting materials while using the least energy input possible.
Previously, unsaturated plant oils have been modified to provide inexpensive monomers which will polymerize rapidly under photoinitiated cationic polymerization conditions. For example, numerous epoxidized triglyceride oils have been utilized as starting materials to make cross-linked polymer networks.
Due to the lack of reactive functionalities, plant oils (e.g., glycerol triesters of unsaturated fatty acids) are not directly amenable to cationic polymerization. However, the olefinic double bonds of these oils can be readily transformed into cationically polymerizable epoxy groups through simple epoxidation reactions. Conventional epoxides and methods for epoxidation have employed oxidation over silver with ethylene, peroxy acids such as peracetic acid in acetic acid solution, organic peroxides, permanganates, chromates, or dehydrochlorination of chlorohydrins with caustic alkenes.
Under ordinary epoxidizing conditions, e.g. utilizing peracetic acid, the yield of epoxidized castor oil is very low. In addition, the use of conventional epoxidizing agents can create safety and environmental concerns. For example, acetic acid is discharged as a polluting by-product.
Catalysts are utilized in the reaction to provide the highest percentage of epoxy groups. Suitable catalysts include heavy metal catalysts such as a tungsten containing heteropolyacid supported on a solid (U.S. Pat. No. 5,430,161 to Brown et al.), catalytic compounds of molybdenum, tungsten, titanium, columbium, tantalum, rhenium, selenium, chromium, zirconium, tellurium, and uranium (U.S. Pat. No. 3,351,635 to Kollar), an acid salt of a peracid of a heavy metal of the group consisting of tungsten and molybdenum (U.S. Pat. No. 2,833,787 to Carlson et al.), a peracid catalyst of the group consisting of the peracids of tungsten, vanadium, and molybdenum (U.S. Pat. No. 2,786,854 to Smith et al.), a compound of a transition metal such as tungsten in the form of tungsten salts or metallo-organic compounds (U.S. Pat. No. 4,197,161 to Friedrich et al.), a catalyst which is a metal of groups IVA, VA, or VIA, preferably molybdenum or tungsten (Belgian Patent 860,776), and a catalyst selected from elementary boron, a mineral or organic derivative of boron, or mixtures thereof (U.S. Pat. No. 4,303,586 to Schirmann et al.).
Other reactions utilize combinations of catalysts or combinations of catalysts and other reagants. These catalysts or catalytic systems include at least one inorganic or organic derivative or compound of mercury and at least one inorganic or organic derivative of transition elements such as tungsten (U.S. Pat. No. 4,026,908 to Pralus et al.), at least one lead compound and at least one compound of a transition metal such as tungsten (U.S. Pat. No. 3,953,480 to Delavarenne et al.), at least one organic tin compound and a second compound selected from molybdenum, tungsten, vanadium, selenium, boron, and mixtures thereof (U.S. Pat. No. 3,806,467 to Watanabe et al.), a transition compound of a metal such as tungsten and a nitrogenous organic base (U.S. Pat. No. 3,778,451 to Poite), tungstic acid and alkaline salts thereof and an onium salt acting as a phase-transfer agent (U.S. Pat. No. 5,336,793 to Gardano et al.), and a catalytic concentration of a peracid of an oxide of a metal from Groups IV, V, VI, or VIII or a peracid of a heteropolyacid and an inorganic or organic alkaline-reacting substance (Great Britain Patent 837,464).
Although these reactions work well for some unsaturated hydrocarbons, success is not universal. One notable exception is castor oil. A review of recent literature contains few references to the preparation of epoxidized castor oil, suggesting that it has found few applications as compared to other epoxidized vegetable oils. In addition, tungsten-based catalysts have previously been recognized as suitable for use with only a few olefins (U.S. Pat. No. 4,973,718 to Buchler et al. and U.S. Pat. No. 4,845,252 to Schmidt et al.).
A need exists for a method for the epoxidation of castor oil and its derivatives using catalysts, either alone or in combination with additional reagents, that produces a high yield of epoxidized castor having sufficient oxirane oxygen content. The epoxidation reaction should avoid side reactions that adversely effect the stability of the oxirane rings by employing a high efficiency catalyst thus permitting shorter contact times. Further, the epoxidized compounds so produced should be particularly well suited for use in cationic photopolymerization reactions to produce three dimensional polymer networks.